The number density, partial pressure or concentration of oxygen may be monitored in a gas or fluid, or in the space above a fluid in a tank or container for various reasons. For example, the concentration of oxygen may be monitored in a fuel tank as part of a fuel inerting system for avoiding or minimizing the possibility of combustion in the fuel tank.
Fuel tank inerting systems have long been used in tactical aircraft, but are typically restricted to open-loop operation. While open-loop inerting systems provide an effective method of inerting the ullage in a fuel tank, such systems are implemented at a cost of higher demands on the propulsion and environmental systems for bleed air and power, and reduced life of the On-board Inert Gas Generation System (OBIGGS). Due to the highly combustible nature of the ullage environment of tactical aircraft, there is an associated requirement that a highly accurate oxygen sensor be used to monitor the ullage to ensure that the concentration of oxygen in the ullage does not exceed a predetermined maximum level. Further, inexpensive, compact oxygen sensors that may be effective for less volatile closed-loop automotive and industrial applications generally are not compatible with the highly combustible ullage environment of a tactical aircraft. This intrinsic safety requirement generally limits OBIGGS systems to open-loop operations, whether applied to military or commercial aircraft.